Process for rendering a steel piece superficially hard and corrosion resistive,piece obtained by the carrying out of this process and use of the latter

ABSTRACT

A PROCESS FOR RENDERING A STEEL PIECE SUPERFICIALLY HARD AND CORRROSION RESISTANT AND THE PRODUCT OF SAID PROCESS. AN INTERMEDIATE LAYER OF AT LEAST ONE PASSIVE METAL SUCH AS CHROMIUM IS APPLIED TO THE STEEL PIECE AS BY REDUCTION OF A CHROMIUM HALIDE DEPOSITED ON THE PIECE, SAID PASSIVE METAL LAYER DIFFUSING AT LEAST PARTIALLY INTO THE BASE METAL, AND A HARDENING LAYER CONSISTING OF A REFRACTORY METAL CARBIDE, NITRIDE OR SILICIDE SUCH AS TITANIUM CARBIDE FOR EXAMPLE, IS THEREAFTER APPLIED CHAMICALLY IN THE GAS PHASE, AS BY TREATMENT OF THE COATED PIECE WITH A GASEOUS DISPERSION OF TITANIUM CHLORIDE IN HYDROCARBONS SUCH AS METHANE, IN THE PRESENCE OF HYDROGEN GAS.

"United States Patent Int. c1.c2ac 11/04 US. Cl. 117-71 M 4 ClaimsABSTRACT OF THE DISCLOSURE A process for rendering a steel piecesuperficially hard and corrosion resistant and the product of saidprocess. An intermediate layer of at least one passive metal such aschromium is applied to the steel piece as by reduction of a chromiumhalide deposited on the piece, said passive metal layer difiusing atleast partially into the base metal, and a hardening layer consisting ofa refractory metal carbide, nitride or silicide such as titanium carbidefor example, is thereafter applied chemically in the gas phase, as bytreatment of the coated piece with a gaseous dispersion of titaniumchloride in hydrocarbons such as methane, in the presence of hydrogengas.

The present invention concerns a process for rendering a steel piecesuperficially hard and corrosion resistant; it also concerns the pieceobtained by the carrying out of this process and the use of the latter.

It is known, for the superficial hardening of steel pieces, tochemically apply, in the gas-phase, onto said pieces a hardening layer,e.g. of titanium carbide, up to thick and having a VNH hardness of 3,000to 4,000 kp./mm.

However, these processes only apply to carbon steel, alloyed or not.These hardened pieces are therefore vulnerable to corrosion attack.

The object of the present invention is to remedy this drawback and toprovide a process for simultaneously hardening such steels and make themcorrosion resistant.

The process of the invention is characterized by the fact that anintermediate or first layer of at least one passive metal is appliedonto the steel piece, said layer difiusing at least partially into thebase metal, and that a hardening layer consisting of a metal carbide,nitride, boride, or silicide is thereafter applied chemically in thegas-phase. The term passive metal is utilized as defining a metalcharacterized by resistance to corrosion-socalled passivity to chemicalattack-and is to be interpreted as such when reference is made theretothroughout the specification and in the claims.

The piece obtained by the carrying out of this process is characterizedby the fact that it is made of a steel base coated with an intermediateor first layer of a passive metal diifusing at least partially into thebase metal, this layer being itself coated with a hardening layerconsisting of a metal carbide, nitride or silicide.

The use of the present process concerns the manufacture of watch-caseelements.

According to a preferred embodiment of the present invention, the abovementioned first layer of at least one passive metal is deposited, as isthe hardening layer, in the gas-phase. The metal used can be chromium,tantalum, aluminum, silicon and nickel, the latter being onlyconsidered, however, when the second or hardening layer 3,796,588Patented Mar. 12,, 1974 does not consist of titanium carbide. In thecase of chromium, experiments have shown that the metal deposited forms,together with the carbon of the steel, chromium carbides which fitparticularly well, in a carbide containing base, for the subsequentdeposition of the titanium carbide hardening layer.

During chromization, the two following reactions occur, one or the otherbeing predominant depending on the reducing agent chosen, hydrogen ormetal:

CrCl +H CT+ 2HCl (I) CrCl +Fe Cr-l-FeCl (II) The starting material is achromium halide, for instance chromium trichloride known in the tradeunder the name of CrCl Hexahydrate, written CrCl in Equations I and IIfor simplification, but which comprises actually chromium halidescorresponding to numbers 2 and 3 of the chromium valences. The reducingagent of reaction I is hydrogen which is progressively added to thechromium halide, whereas in Equation 11, the reducing agent, which isgenerally identical with the base metal, is iron.

According to the conditions chosen, temperature, pressure, gasconcentration, nature of the base material, etc., the chromium layerwhich forms on the surface of the sample diffuses partially or entirelyinto it.

Deposition temperatures will be preferentially above 800 C. for having ahigh enough chromium chloride pressure.

Large diffusion zones can be obtained in steels containing less than0.05% C. Steels which are ferrite rich or which contain elements thattake part to the ferrite phase or easily bond with carbon, e.g. Cr, Mo,W, V, Ta, Ti, Zr, Al, Si and Nb favor difiusion. For instance, chromiumdifiusion layers form easily on stainless steel.

By adding hydrogen in excess to chromium halides, the exchange reactionH is eliminated and the deposition reaction I is favored.

By the use of high carbon steels (C content) 0.2%), austenite steels orsteels containing elements taking part to the formation of the austenitephase, e.g. Ni, Mn, Cu and N, the diffusion is strongly decreased.

Steels having standard numbers (Werkstofinummer) 1,2000 to 1,3500 build,when chromium is deposited, chromium carbide layers having good adhesionand no pores, and being hard and wear resistant.

Gaseous 'HCl can also be reacted with Cr metal to form CrCl which willbe reduced thereafter to a layer of Cr metal. A :1 to 20:1 mixture ofargon and HCl is reacted, at a temperature above 800 C. (preferentiallyat 850 C.) on granulated or pulverized chromium evenly dispersed in acontainer entirely made of alumina. The equilibrium reaction I appliesthere, but in the opposite direction.

In place of HCl, elementary chlorine can be used for making CrClaccording to the following equation:

For the deposition of chromium, the mixture obtained above, Cr/Ar/H/remainder of HCl, is allowed to react with the substrate, eitherdirectly or diluted with H Large volumes of gas under atmospheric orreduced pres sure (preferentially 50 to 100 torr) are used foreliminating, as quickly as possible, the gaseous products which arenormally HCl and FeCl;.

It is also possible to produce the first layer with several metals, e.g.by the deposition of a first layer of Si and a second with Cr.

Care should be exercised so that the first layer is not softer than thebase-material.

The final hardening coating, that is the second layer, which is known aswell as the methods for obtaining it,

will be also obtained chemically in the gas-phase. It will consist of acarbide, nitride or silicide, of a metal from Groups III to VI of thePeriodical Table, e.g. tantalum or titanium carbide.

In the case of, for instance, titanium carbide, a titanium compound suchas TiCl is progressively evaporated from an evaporator for liquids andcontinuously added to a gaseous flow of hydrocarbons. Such are methane,acetylene or ethylene, which are added in their normal gaseous state,and dicyclopentadiene which must be vaporized beforehand.

A 1:1:100 to 5:5 :100 gaseous mixture of CH TiCl and H, can be used onthe base-material in an Inconel reactor, avoiding carefully all ironparts.

From the equation A: H5 CH4 'IiCl, T 4HC1 (III) there is an increase inthe volume of the products that is the reason for which working under areduced pressure of 10-100 torr (temperature 800900 C.) is preferable.

The said second layer can also be made of an alumina layer or, further,of a composite layer, e.g. of chromium carbide and titanium carbide orof titanium carbide and nitride for having color hues varying fromgolden to silvered.

The present process permits obtaining any hard and corrosion resistantpieces, particularly Watch-case elements such as middle parts, bezels,protective caps for middle parts, bezels, etc.

What we claim is:

1. A watch element comprising a shaped steel substrate having (an atleast partially diffused) a gas-phase chemically deposited chromiumsulface layer (of at least one corrosion resistant material havingmetallic characteristics selected from the group consisting of chromium,tantalum, aluminum, silicon, and nickel) at least partially diffusedinto said steel substrate, and a gas-phase chemically deposited secondlayer (formed on said first layer) on said chromium layer, said secondlayer (selected from the .4 group consisting of titanium carbide,titanium nitride, a mixture of titanium carbide and titanium nitride,chromium carbide, tantalum carbide, nickel boride and aluminum oxide)comprising titanium carbide.

2. The watch element as claimed in claim 1 wherein said first layer isthe reduction product obtained by reacting chromium trichloride with areducing agent.

3. The watch element as claimed in claim 1 in which said first layer isat least as hard as the substrate material.

4. The watch element as claimed in claim 1 in which said first layer isthe reduction product of the reaction product of one of gaseous HCl andchlorine gas on chromium metal applied directly to said substrate.

References Cited UNITED STATES PATENTS 3,690,062 9/ 1972 Kasai et al.58-88 R 3,368,914 2/1968 Darnell et al. 117-71 M 2,756,162 7/1956 Garneret al 117-71 M X 3,029,162 4/1962 Samuel et al 117-107 2,872,350 2/1959Homer et al 117-106 C 2,836,514 5/1958 Miinster et al. 117135.1 X3,586,614 6/1971 Boggs et al. 117-71 M X 3,031,331 4/1962 Aves, Jr. etal 117-71 M 3,496,010 2/1970 Bracken et al 117-69 X 3,642,522 2/ 1972Gass et al. 117-106 C 3,242,664 3/ 1966 Lederrey 29-179 X 3,669,695 6/1972 Her et a1 5 8-88 R X OTHER REFERENCES Powell, C. F., et al.: VaporPlating. John Wiley & Sons, Inc., N.Y., 1955. pp. 47, 48, 140 and 141.

RALPH S. KENDALL, Primary Examiner US. Cl. X.R.

117-106 A, 106 C, 107.2 R; 29-195; 58-88 R UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION PATENT NO. I 3,796,588

DATED I March 27, 1974 INVENTOR(S) I HANS E. HIN'IERMANN and WERNER'HANNI It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

Column 3,line 33, change "sulface" to -surface-; Column 3, lines '31 to39 and Column 4, lines 1 to 4, all material within parentheses and theparentheses should be deleted,

Claim 1 should read as follows:

l. A watch element comprising a shaped steel substrate having-agas-phase chemically deposited chromium surface layer at least partiallydiffused into said steel substrate, and a gas-phase chemically depositedsecond layer on said chromium layer, said second layer comprisingtitanium carbide.

r Signed and sealed this 1st day of July 1975.

(SEAL) Attest:

C. Z IARSHALL DANN RUTH C. MASON Commissioner of Patents AttestingOfficer and Trademarks v UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION PATENTNOT: 3,796,588-

.DATED 3 March 27,1974

lNVENTOR(5) I HANS E. HINTERMANN and WERNER 'HANNI It is c ertifi edthat error appears in the above-identified patent and that saidLettersPatent are hereby corrected as shown below:

Column 3,line 33, change "sulface" to -surface-; Column 3, lines '31 to39 and Column 4, lines 1 to 4, all material within parentheses and theparentheses should be deleted, Claim 1 should read as follows:

l. A watch element comprising a shaped steel substrate having agas-phase chemically deposited chromium. surface layer at leastpartially diffused into said steel substrate, and a gas-phase chemicallydeposited second layer on said chromium layer, said second layercomprising titanium carbide.

' Signed and sealed this 1st day of July 1975.

(SEAL) Attest: v

v C. MARSHALL DANN RUTH C. MASON Commissioner of Patents AttestingOfficer and Trademarks

